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Slip of the Chameleon's Tongue

Chameleons' sticky tongues lash out at unsuspecting bugs with amazing speed. For almost a century, zoologists thought they had this feat of bioengineering figured out. But a new study shows they missed the most important bit.

Zoology textbooks explain that the chameleon's ballistic tongue, stretchable to almost twice the animal's length, is powered by a large accelerator muscle. The muscle lengthens as it squeezes down on the tongue bone, a stiff cartilage in the core of the tongue, that it envelops. But in a paper to be published in the Proceedings of the Royal Society of London (Series B), two morphologists studying the lizard's feeding behavior show that hidden forces are at work.

Jurriaan de Groot of Leiden University and Johan van Leeuwen of Wageningen University, the Netherlands, took high-speed x-ray films of chameleons catching prey. The films, capturing 500 frames per second, showed that the tip of the tongue accelerates with up to 50 g, five times the acceleration that a fighter jet can reach. Factoring in the mass of the tongue tissues, the researchers calculated that the accelerator muscle is nowhere near powerful enough to do all that work.

The researchers then dissected several chameleon tongues and revealed a hitherto unknown set of at least 10 slippery sheaths between the accelerator muscle and the tongue bone. The sheaths, which are attached to the tongue bone at the end closest to the chameleon's mouth, proved to contain spirally wound protein fibers that are squeezed out of shape when the accelerator contracts, storing energy like a stretched rubber band. When the stressed and lengthened sheaths reach the rounded end of the tongue bone, they will slip off simultaneously and contract with force, while constantly pushing the tongue off the rod. Just after they have slipped of the tongue bone, the sheaths, which are arranged in a staggered fashion, fall apart like the tubes of a telescope, thus allowing the tongue its maximum stretch. "It is a sort of telescopic catapult," says van Leeuwen.

The novel mechanism fires the imagination of other zoologists. David Wake of the University of California, Berkeley, who has been working on the mechanics of salamanders tongues, thinks the chameleon study is "a very exciting piece of science." He adds that the same sliding spring mechanism may well be at work in other vertebrates with ballistic tongues, such as salamanders and certain birds.